1. Field of the Invention
The present invention relates to an optical device with a fixed optical fiber array having one or more optical fibers and to a method of manufacturing such an optical device, and more particularly to an optical device suitable for monitoring signal light while the signal light is propagated through an optical fiber array and to a method of manufacturing such an optical device.
2. Description of the Related Art
For present optical communication technology, it is important to monitor communication quality. Particularly, monitoring of optical output plays an important role in the field of wavelength multiplex communication technology.
In recent years, there have been growing demands for smaller size, higher performance and lower cost optical output monitoring technology.
Heretofore, a technique has been proposed and disclosed in Japanese Laid-Open Patent Publication No. 2001-264594, for example. According to this proposed technique, as shown in FIG. 13, an optical fiber 202 is placed in a V-shaped groove in a glass substrate 200, and then a slit 204 is formed in the glass substrate 200 obliquely to the optical axis, in and across the optical fiber 202. A filter member 206 is inserted into the slit 204, wherein a gap therebetween is filled with an ultraviolet-curable resin (adhesive) 208. The filter member 206 has a substrate 210 and a multilayer film 212 formed on a principal surface of the substrate 210. The multilayer film 212 is designed to match the refractive index of the substrate 210 and the refractive index of the resin 208 that fills the slit 204.
When signal light 214 is propagated through the optical fiber 202, a light component (reflected light) 216 divided by the filter member 206 is extracted out of the cladding. The reflected light 216 is detected by a photodetector, for example, to monitor the signal light 214.
The structure of the optical path for applying the divided light to the photodetector greatly affects the monitored characteristics of the signal light. It is desirable to simplify the medium that exists within the optical path of the divided light.
The conventional optical device has various interfaces, which are produced by the resin provided within the slit, the cladding layer, the fixing adhesive, a further resin for achieving refractive index matching with the photodetector (i.e., a refractive index matching resin), and the light-detecting surface of the photodetector. Further, since the slit is formed obliquely, the divided light from the filter member is emitted obliquely therefrom. Therefore, a PDL (Polarization Dependent Loss) occurs.
The dependency of the PDL on the interfaces increases as the number of interfaces in the optical path of the divided light increases. Particularly, at the interface where light is emitted from the cladding layer of the optical fiber, the refractive index of the cladding layer material (i.e., glass) and the refractive index of the resin filling the optical path must be substantially equal to each other. However, since the adhesive (fixing adhesive) for securing the optical fiber within the V-shaped groove creeps onto the optical fiber during the manufacturing process, the fixing adhesive also exists inside the optical path of the divided light.
The refractive index of the fixing adhesive and the refractive index of glass differ widely from each other. Since fixing adhesives having refractive indexes close to the refractive index of glass do not exist on the market, it is difficult to equalize the refractive index within the optical path of the divided light to that of the refractive index of glass. Therefore, the PDL undesirably increases.
Furthermore, since the spot diameter of the divided light increases depending on the distance of the optical path, the distance from the optical fiber to the photodetector should be reduced in order to increase the light detection efficiency of the photodetector. However, the fixing adhesive within the optical path of the divided light makes it difficult to reduce the distance from the optical fiber to the photodetector.
One approach would be to remove the fixing adhesive that has crept onto the optical fiber. However, such a method would require a new process step for removing the fixing adhesive. Depending on how the fixing adhesive is removed, the surface of the optical fiber may become scratched, tending to increase the PDL. Also, damage to the upper surface of the optical fiber may impair reliability.
If a refractive index matching resin is provided on the optical fiber, then depending on the viscosity of the resin, the resin may flow out due to thermosetting conditions during the manufacturing process and due to aging, resulting in air being introduced into the optical path of the divided light. Such introduced air is liable to lead to failures when performing monitoring functions.